VPD Policy Issue.

Hi , I am using 11.2.0.3.0 version of oracle.

I am trying below VPD policy function on one of my column and i am expecting the column value to be shown as null when the table is being queried from USER1 and USER2. But i am not able to get it, its showing all the text values.
(Note-I dont want to put the restriction on SCHEMA(USER1,USER2) level for all objects using default 'EXEMPT ACCESS POLICY', so i have revoked the same from USER1,USER2 and trying to achive the same using below policy function.)